By Deandra Grant, J.D., M.S. (Pharmaceutical Science), ACS-CHAL Forensic Lawyer-Scientist
If you are arrested for DWI in Texas and the officer believes you are impaired by a drug rather than alcohol, your case is fundamentally different from an alcohol DWI and, in many respects, it is more defensible. The reason is simple: Texas has no per se limit for drugs. There is no number (ex. no drug equivalent of the 0.08 BAC threshold) that makes you guilty. The prosecution must prove that you were actually impaired at the time of driving, not merely that a substance was present in your body. And proving impairment from drugs is far harder than proving impairment from alcohol, because the science is less settled, the testing is less reliable, and the relationship between drug concentration and impairment is far more complex.
DUID (driving under the influence of drugs) prosecutions are increasing across Texas as marijuana use grows, prescription drug use rises, and law enforcement expands its use of Drug Recognition Expert (DRE) officers. But the forensic science underlying these cases has not kept pace with the prosecution’s ambitions. The DRE protocol has significant validation problems. Blood tests detect the presence of drugs but often cannot establish impairment at the time of driving. And the pharmacokinetics of most drugs are so variable from person to person that extrapolating backward from a blood draw result to a driving-time drug level is scientifically unreliable.
This article explains how drug-impaired driving cases are investigated and prosecuted in Texas, where the science breaks down, and how an informed defense challenges every step of the process.
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The Legal Framework: No Per Se Limit
Under Texas Penal Code §49.04, a person commits DWI by operating a motor vehicle in a public place while intoxicated. Intoxication is defined under §49.01(2) in two ways:
- Per se intoxication: Having a blood alcohol concentration of 0.08 or more.
- Impairment intoxication: Not having the normal use of mental or physical faculties by reason of the introduction of alcohol, a controlled substance, a drug, a dangerous drug, a combination of two or more of those substances, or any other substance into the body.
For alcohol cases, the prosecution can rely on either prong. Id the BAC is 0.08 or higher, the defendant is legally intoxicated regardless of whether they appeared impaired. But for drug cases, only the impairment prong is available. There is no statutory threshold for any drug that means no concentration of THC, no concentration of benzodiazepines, no concentration of opioids, no concentration of any controlled substance that automatically constitutes intoxication under Texas law.
The prosecution in a DUID case must prove two things: (1) that a drug was in the defendant’s system, and (2) that the drug actually caused the defendant to lose the normal use of their mental or physical faculties at the time of driving. Proving the first is usually straightforward in that a blood test will detect the substance. Proving the second is where DUID prosecutions can fall apart, because the science connecting drug presence to drug impairment is far weaker than the science connecting BAC to alcohol impairment.
The Drug Recognition Expert Protocol: 12 Steps, Seven Drug Categories, and Significant Limitations
When an officer suspects drug impairment (typically because the driver appears impaired but there’s no odor of alcohol or a breath test shows little or no alcohol) the officer may call a Drug Recognition Expert (DRE) to conduct an evaluation. The DRE program is administered jointly by NHTSA and the International Association of Chiefs of Police (IACP). DRE officers undergo specialized training in recognizing the symptoms of impairment from seven categories of drugs.
The Seven Drug Categories
The DRE protocol classifies all impairing substances into seven categories, each with a characteristic “symptom matrix” of physiological indicators:
- CNS Depressants (alcohol, benzodiazepines, barbiturates, GHB): Slowed reactions, uncoordinated movements, slurred speech, drowsiness, horizontal gaze nystagmus (HGN) present, pupils normal to dilated.
- CNS Stimulants (cocaine, methamphetamine, amphetamines, MDMA): Hyperactivity, rapid speech, elevated pulse and blood pressure, dilated pupils, body tremors, teeth grinding, increased body temperature.
- Hallucinogens (LSD, psilocybin, mescaline): Disorientation, paranoia, hallucinations, dilated pupils, elevated pulse, synesthesia, unpredictable behavior.
- Dissociative Anesthetics (PCP, ketamine, dextromethorphan): Blank stare, HGN present, elevated blood pressure, decreased pain response, rigid muscle tone, impervious to pain.
- Narcotic Analgesics (heroin, fentanyl, oxycodone, hydrocodone, morphine): Droopy eyelids, constricted pupils (miosis), drowsiness, slow and deliberate speech, depressed vital signs, “on the nod” behavior.
- Inhalants (toluene, gasoline, nitrous oxide, aerosols): Disorientation, slurred speech, possible residual odor, HGN present, flushed face, watery eyes.
- Cannabis (marijuana, THC, hashish): Reddened conjunctiva (bloodshot eyes), dilated pupils in dim light but possibly normal in room light, elevated pulse, impaired time estimation, eyelid tremors, possible greenish coating on tongue, distinctive odor.
The 12-Step Evaluation
The DRE evaluation follows a standardized 12-step protocol:
- Step 1: Breath alcohol test (to rule out or quantify alcohol)
- Step 2: Interview of the arresting officer
- Step 3: Preliminary examination (first pulse, general demeanor, speech, pupil size in ambient light)
- Step 4: Eye examinations (HGN, vertical gaze nystagmus, lack of convergence)
- Step 5: Divided attention tests (modified Romberg, walk-and-turn, one-leg stand, finger-to-nose)
- Step 6: Vital signs (blood pressure, pulse, body temperature)
- Step 7: Dark room examinations (pupil size in near-total darkness, direct light, and indirect light; estimation of pupil size in millimeters using a pupilometer)
- Step 8: Muscle tone examination
- Step 9: Injection site examination
- Step 10: Suspect’s statements and interrogation
- Step 11: DRE’s opinion (which drug category or categories, if any)
- Step 12: Toxicological examination (blood or urine sample collected for laboratory analysis)
The DRE’s conclusion is an opinion which means it’s a subjective determination by a police officer, not a scientific measurement. The DRE examines physiological indicators, compares them against the symptom matrix, and renders a judgment about which drug category is causing the observed impairment. The blood or urine sample collected in Step 12 is sent to a laboratory for analysis, and the lab results either confirm or fail to confirm the DRE’s opinion.
Where the DRE Protocol Breaks Down
The DRE protocol is presented to courts as a validated, standardized procedure. The defense must challenge that characterization at every level:
- The validation research is methodologically weak. The studies that purport to validate the DRE protocol suffer from significant design limitations. The original LAPD validation study and subsequent laboratory studies were not double-blind. That means the DRE evaluators knew the subjects had been given drugs, which introduces expectation bias. Sample sizes were small. The studies were conducted on subjects who had been administered a single known drug in a controlled setting which is a scenario that bears little resemblance to real-world DWI cases where the defendant may have taken multiple substances, may have tolerance from chronic use, and may have medical conditions that mimic impairment indicators.
- Accuracy rates are lower than commonly claimed. The DRE program frequently cites overall accuracy rates in the range of 80–90%. But these numbers aggregate all drug categories, and accuracy varies dramatically by category. DRE accuracy is highest for opioids (distinctive miosis) and lowest for cannabis and CNS depressants, which are the most commonly encountered drug categories in real-world DUID cases. When polydrug use is involved (the most common scenario in practice) accuracy drops further because the symptom matrices overlap and contradict each other.
- Many DRE indicators have innocent explanations. Dilated pupils can be caused by medications, eye drops, low ambient light, stress, or naturally large pupil size. Elevated pulse and blood pressure can be caused by the stress of a traffic stop, physical exertion, caffeine, anxiety, or medical conditions (hypertension, hyperthyroidism). Bloodshot eyes can be caused by allergies, dry air, contact lenses, fatigue, or wind exposure. Eyelid tremors can be caused by fatigue or caffeine. The DRE protocol does not adequately control for these alternative explanations and the officer is trained to identify indicators consistent with drug impairment, not to rule out alternative causes.
- The DRE is a police officer, not a medical professional. The DRE receives approximately 72 hours of classroom training and conducts 12 supervised evaluations to earn certification. They do not hold medical degrees, nursing credentials, or any clinical license. They are trained to take vital signs and observe physiological indicators, but their training does not include the pharmacology, pharmacokinetics, or clinical differential diagnosis skills that would be required to reliably distinguish drug impairment from medical conditions, fatigue, or the effects of legally prescribed medications.
- The symptom matrix is not diagnostic. The symptom matrix used by DREs is a chart of expected physiological effects for each drug category. But the matrix describes typical responses, not universal ones. Individual variation in drug response is enormous, particularly for cannabis and CNS depressants. A chronic cannabis user may exhibit none of the expected indicators. A patient stabilized on a prescription opioid may have developed complete tolerance to the CNS-depressant effects while still showing miosis (constricted pupils). The DRE has no reliable way to account for tolerance, individual pharmacokinetic variation, or the interaction effects of multiple substances.
The Blood Test Problem: Presence Is Not Impairment
After the DRE evaluation, a blood sample is collected and sent to a forensic laboratory for toxicological analysis. The lab identifies which substances are present and, ideally, quantifies their concentration. In many DUID cases, the blood test result is the prosecution’s most important piece of evidence. But it is also the most misunderstood.
The Metabolite Problem
Many drugs are rapidly metabolized by the body into inactive metabolites that remain detectable in blood and urine long after the pharmacological effects have ended. The most significant example in Texas DUID cases is THC and its metabolites.
When a person uses marijuana, the active compound Δ⁹-THC enters the bloodstream, crosses the blood-brain barrier, and produces psychoactive effects. THC is then metabolized in the liver, first to 11-hydroxy-THC (11-OH-THC, which is pharmacologically active) and then to 11-nor-9-carboxy-THC (THC-COOH or carboxy-THC, which is pharmacologically inactive). Active THC has a very short half-life in blood in that concentrations drop rapidly within 1–3 hours after use. But carboxy-THC can remain detectable in blood for days to weeks after last use, particularly in chronic users, because it is stored in fat tissue and released slowly.
This creates a fundamental problem for the prosecution: a positive blood test for carboxy-THC proves prior marijuana use but does not prove impairment at the time of driving. A person who last used marijuana three days ago and has no psychoactive effects whatsoever will still test positive for carboxy-THC. If the lab reports “THC metabolite detected” without specifying whether the result reflects active THC or inactive carboxy-THC, the result is scientifically useless for establishing impairment.
The defense must demand that the laboratory report specify exactly which analyte was detected: parent Δ⁹-THC, 11-OH-THC, or THC-COOH. If only carboxy-THC was detected, the prosecution cannot establish that the defendant was impaired by marijuana at the time of driving. If active THC was detected, the concentration must be evaluated in the context of the defendant’s use pattern (occasional vs. chronic), the time elapsed between driving and the blood draw, and the known pharmacokinetic profile of THC.
The Same Problem Across Drug Classes
The metabolite problem is not unique to cannabis. It applies across multiple drug classes that are commonly encountered in Texas DUID cases:
- Benzodiazepines: Diazepam (Valium) has an active metabolite (nordiazepam) with a half-life of 40–100 hours. Alprazolam (Xanax) has a half-life of 6–12 hours, but detectable metabolites persist longer. A positive blood test for a benzodiazepine or its metabolite does not establish impairment at the time of driving.
- Opioids: Hydrocodone, oxycodone, and morphine are metabolized to various compounds that may persist in blood after the analgesic and sedative effects have worn off. Codeine is metabolized to morphine, which means a person taking prescribed codeine cough syrup can test positive for morphine.
- Amphetamines: Methamphetamine has a half-life of approximately 10–12 hours, meaning detectable concentrations may persist for 24+ hours after the stimulant effects have subsided. As discussed in our separate article on l-methamphetamine vs. d-methamphetamine, the lab must also perform chiral analysis to determine whether the detected methamphetamine is the illicit d-form or the legal l-form found in Vicks VapoInhaler.
- Prescription sleep medications: Zolpidem (Ambien) has a short half-life (2–3 hours) but can produce next-morning impairment in some individuals, particularly at higher doses and in women (who metabolize zolpidem more slowly). The FDA has issued specific warnings about driving the morning after taking extended-release zolpidem. This creates a complex defense scenario: the defendant may be impaired by a legally prescribed medication taken as directed.
Retrograde Pharmacokinetics: Why Back-Calculation Doesn’t Work for Drugs
In alcohol DWI cases, the prosecution sometimes uses retrograde extrapolation to estimate the defendant’s BAC at the time of driving based on the BAC at the time of testing. This is already scientifically problematic for alcohol (as we discuss in our morning-after DWI article). For drugs, it is essentially impossible.
Alcohol follows relatively predictable, zero-order elimination kinetics. It is metabolized at a roughly constant rate regardless of concentration (approximately 0.015% BAC per hour, with individual variation). Most drugs follow first-order or mixed-order kinetics, meaning the rate of metabolism depends on the drug’s concentration, the enzyme systems involved, the individual’s liver function, their genetic profile of metabolizing enzymes (CYP2D6, CYP3A4, CYP2C19, and others), concurrent medications that may inhibit or induce those enzymes, food intake, and numerous other variables.
The practical result: there is no scientifically reliable way to extrapolate backward from a drug blood level at the time of testing to estimate the drug level at the time of driving. The prosecution cannot use a blood draw result obtained 60–90 minutes after a traffic stop to calculate what the defendant’s drug concentration was when they were actually behind the wheel. Any attempt to do so is speculative, and the defense should object to it as scientifically unreliable.
Prescription Medication DUID: The Fastest-Growing Category
An increasing proportion of DUID cases in Texas involve defendants who are taking legally prescribed medications as directed by their physicians. These cases raise unique defense issues:
The Tolerance Problem
Patients who take benzodiazepines, opioids, or other CNS-depressant medications on a chronic, stable basis develop pharmacological tolerance which means their bodies adapt to the drug’s effects, and they experience less sedation, less cognitive impairment, and less psychomotor impairment at the same blood concentration that would impair a drug-naïve individual. A patient who has been taking alprazolam 1 mg twice daily for two years may function normally at a blood concentration that would severely impair someone taking the drug for the first time.
The DRE protocol does not account for tolerance. The DRE evaluates physiological indicators against a symptom matrix that reflects the expected effects in a non-tolerant individual. A tolerant patient may exhibit some indicators (miosis from opioids, for example) without any meaningful impairment of driving ability. The DRE has no training in pharmacological tolerance and no reliable method for distinguishing a tolerant patient from a non-tolerant user.
Therapeutic Levels vs. Impairment Levels
For many prescription medications, the blood concentration that produces therapeutic benefit overlaps significantly with the concentration that the prosecution will argue constitutes impairment. A patient taking a prescribed dose of hydrocodone for chronic pain may have a blood concentration of 20–40 ng/mL which is a therapeutic range that is consistent with both effective pain management and normal daily functioning. The prosecution will point to the blood test and say “opioid detected.” The defense must explain to the jury that the presence of a prescribed medication at a therapeutic level does not equal impairment, particularly in a tolerant patient.
The Prescriber’s Duty and the Patient’s Responsibility
Under Texas law, a person can be convicted of DWI for impairment caused by a legal, prescribed medication. The law does not require the substance to be illegal. If a prescription drug causes the defendant to lose the normal use of their mental or physical faculties, the DWI statute applies.
However, the defense can present evidence that the defendant was taking the medication as prescribed, that the prescriber did not warn against driving (or affirmatively stated that driving was safe), that the defendant had been functioning normally on the medication for an extended period, and that the defendant had no reason to believe the medication would impair their driving on this occasion. Pharmacy records, prescriber testimony, and the medication’s package insert (which may or may not include driving warnings) are all relevant evidence.
Case Results
Field Sobriety Tests: Designed for Alcohol, Applied to Drugs
The three Standardized Field Sobriety Tests (HGN, Walk-and-Turn, and One-Leg Stand) were developed and validated by NHTSA for the detection of alcohol impairment. They were not developed for and have not been validated for the detection of drug impairment.
This is a critical distinction that most DUID defendants do not understand: the SFSTs you were asked to perform on the side of the road were not designed to detect the type of impairment the officer was looking for. The NHTSA validation studies that support the SFSTs tested subjects who had consumed alcohol, not drugs. The clue systems (two clues on Walk-and-Turn, two clues on One-Leg Stand, four clues on HGN) were calibrated against known BAC levels, not drug concentrations.
HGN (the eye test where the officer has you follow a stimulus with your eyes) is the SFST most commonly cited in drug cases. HGN is associated with alcohol, CNS depressants, dissociative anesthetics, and inhalants. But HGN is not associated with cannabis, stimulants, or narcotic analgesics which are three of the most common drug categories in real-world DUID cases. An officer who observes HGN clues and concludes the defendant is impaired by marijuana or methamphetamine is misapplying the science.
Deandra Grant is a trained SFST Instructor and holds the credentials to testify as to the proper administration, scoring, and limitations of every standardized test. When the prosecution’s case relies on SFST performance as evidence of drug impairment, the defense must expose the foundational problem: these tests were not built for this purpose.
Suppression: Article 38.23 and the Drug DWI
The suppression analysis in a DUID case follows the same framework as any DWI case under CCP Article 38.23 which is Texas’ exclusionary rule that has no good-faith exception. Evidence obtained in violation of any provision of the Constitution or laws of Texas or the United States is inadmissible.
Common suppression targets in DUID cases include:
- The initial stop: Was there reasonable suspicion for the traffic stop? If the driving behavior was consistent with normal driving (or with the effects of fatigue, distraction, or road conditions rather than impairment), the stop may be challengeable.
- The extension of the stop: Once the officer determined there was no significant alcohol on the breath, did the officer have reasonable suspicion of drug impairment sufficient to extend the stop for a DRE evaluation? The transition from “the breath test is negative” to “I’m calling a DRE” requires articulable facts supporting drug impairment and not merely a hunch.
- The blood draw warrant: If the defendant refused to provide a blood sample voluntarily, the officer must obtain a search warrant. The warrant affidavit must contain facts establishing probable cause to believe the defendant’s blood contains evidence of intoxication. Boilerplate warrants that do not describe drug-specific indicators are vulnerable to challenge.
- The blood draw procedure: The blood draw must be conducted by qualified personnel (phlebotomist, nurse, physician, or other authorized person) using proper procedures and equipment. Chain of custody from the draw site to the laboratory must be documented. Contamination, improper storage (failure to refrigerate, use of incorrect preservative tubes), and breaks in the chain of custody are all grounds for suppression or exclusion of the result.
Defense Strategies Specific to DUID Cases
1. Demand Specificity in the Blood Test Results
The defense must obtain full laboratory discovery, not just a summary. Determine exactly which analytes were tested for, which were detected, and at what concentrations. For cannabis cases, insist on distinguishing parent Δ⁹-THC from inactive carboxy-THC. For methamphetamine cases, was a chiral analysis done to determine whether the detected methamphetamine is the illicit d-form or the legal l-form? For prescription medication cases, compare the detected concentration against published therapeutic ranges.
2. Challenge the DRE’s Opinion
The DRE’s opinion is exactly that — an opinion. Cross-examine the DRE on: their training (72 hours of classroom instruction and 12 supervised evaluations), their accuracy rate in prior evaluations (request their evaluation logs and compare their predictions against actual lab results), whether they considered alternative explanations for each physiological indicator, whether their opinion is consistent with or contradicted by the laboratory results, and whether the symptom matrix they relied on accounts for tolerance, polydrug use, and individual variation.
3. Challenge the Retrograde Extrapolation
If the prosecution attempts to extrapolate backward from the blood draw result to estimate the drug concentration at the time of driving, object. The pharmacokinetics of most drugs do not support reliable back-calculation. Retain a pharmacology or toxicology expert to testify that the individual variation in drug metabolism is too great to support a reliable estimate. Grant’s pharmaceutical science training includes the pharmacokinetic principles (absorption, distribution, metabolism, elimination) that underlie this challenge.
4. Present the Tolerance Defense
For prescription medication cases, present evidence of the defendant’s treatment history, dosing regimen, duration of use, and prescriber’s assessment of the defendant’s ability to drive. Medical records, pharmacy records, and prescriber testimony establish that the defendant was taking a legal medication as directed and had developed tolerance to its impairing effects.
5. Attack the Gap Between Presence and Impairment
The prosecution must bridge the gap between “drug detected in blood” and “drug caused impairment at the time of driving.” Every defense strategy in a DUID case is built around widening this gap: the drug was present but the metabolite was inactive, the drug was present but at therapeutic levels in a tolerant patient, the drug was present but the defendant’s driving was normal, the drug was present but the DRE’s indicators have innocent explanations, the drug was present but the back-calculation is unreliable. If the prosecution cannot bridge the gap with reliable scientific evidence, the evidence is insufficient to prove intoxication beyond a reasonable doubt.
6. Suppress the Evidence
If the stop was unlawful, the extension was unjustified, or the blood draw was improperly obtained, suppress everything. Under Article 38.23, there is no good-faith exception. If the warrant was defective, the blood result is inadmissible. Without the blood result, the prosecution is left with the DRE’s subjective opinion and the officer’s observations, neither of which may be sufficient to prove intoxication beyond a reasonable doubt.
DUID Defense at Deandra Grant Law
Drug-impaired driving cases require a defense team that understands both the law and the science. The law is Texas-specific: §49.04, §49.01, Article 38.23, the ALR process, and the case law governing DRE testimony and drug blood evidence. The science is pharmacology: drug metabolism, pharmacokinetics, the relationship between blood concentration and impairment, the metabolite problem, the tolerance defense, and the limitations of the DRE protocol.
Deandra Grant brings both. Her Master’s Degree in Pharmaceutical Science (Forensic Science Concentration) from the University of Florida College of Pharmacy and Graduate Certificate in Forensic Toxicology from UF’s College of Veterinary Medicine give her the academic training to understand drug metabolism at the molecular level. She is recognized nationally as an authority in this field. Her ACS-CHAL Forensic Lawyer-Scientist designation and SFST Instructor training complete the forensic science credentials that DUID cases require.
Doug Huff’s ACS-CHAL credential and digital forensics training extend the defense capability to dashcam and bodycam analysis (critical for evaluating whether the defendant’s driving behavior was actually consistent with impairment), phone records, and GPS evidence.
If you have been arrested for DWI based on suspected drug impairment in North Texas, call (214) 225-7117 or visit texasdwisite.com. In a DUID case, the prosecution’s evidence looks scientific. Our job is to show the jury what the science actually says.
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